Remote sensing involves the acquisition of information or data around a distant object or system without being in physical contact with it. Most remote sensing instruments are designed to analyze the characteristics of the electromagnetic spectra reflected by objects (their spectral signatures) to allow one to determine some of the objects' properties. Human vision uses the same principle when using color (the sensation produced when light of different wavelengths falls on the human eye) to identify objects. The sensors used in remote sensing, however, make it possible to broaden the field of analysis to include parts of the electromagnetic spectrum that are well beyond visible light such as ultraviolet (<0.3 μm), visible (0.4–0.7 μm), near-infrared (0.7–1.5 μm) and thermal infrared (up to 1000 μm or 1 mm) ranges.
Today, remote sensing technology is used in a variety of applications in fields such as hydrology, geology, environment, transportation, ecology, and earthquake engineering. One particular application involves airborne imaging where remote sensors are placed on-board aircraft to make observations and images of the Earth. These airborne remote sensor systems generally use either a mechanical scanning technique or a linear array, along with aircraft motion, to acquire terrestrial imagery.
One drawback to using current airborne imaging techniques is the inferior geometric fidelity in image quality since the two-dimensional spatial images captured by the remote sensors are not acquired at the same instant. During airborne imaging, each image scene that is collected from a target area consists of a two-dimensional grid of discrete cells, each of which is referred to as a pixel. For scanning sensors, adjacent pixels are acquired at different times, while for linear array sensors, adjacent rows of pixels are acquired at different times. Attitude data meanwhile are sampled once per scan revolution. Consequently, any changes in the direction of the aircraft's velocity or attitude results in geometric distortions for different regions within the two-dimensional image. Also, sufficient information is not available to obtain accurate records of the sensor's location or its attitude parameters at the appropriate instant. Therefore, the collected data requires sophisticated and expensive post-mission processing to improve upon the geometric fidelity and to achieve a positioning accuracy that meet the user's requirement.
Another drawback to current airborne imaging is that the remote sensors mounted to the aircraft have to be calibrated in order to accurately obtain the absolute geophysical coordinates of the remote sensing data. During normal operation, the remote sensing data acquired during the flight must be transferred from the original mission medium to a working medium. The remote sensing data is then processed in a centrally located data processing center before it is distributed to end users. To obtain the desired level of accuracy on the absolute geophysical coordinates, each user has to perform additional image processing. This includes sophisticated and extensive ground processing and, in many cases, collecting supporting data on ground control points before the absolute geophysical coordinates on any feature in the terrestrial imagery can be obtained. No accurate absolute geophysical coordinate information, suitable for medium and large scale mapping applications, of any terrestrial features in an image scene is available on the original mission medium.
One method of calibrating a remote sensor is to place calibration targets on the target area that is to be sensed. Panels made of cloth have been used as calibration targets but are expensive, difficult to handle, require intensive effort to lay out in a field, are easily damaged, and usually must be gathered up after the calibration exposure is completed. In addition, deploying calibration targets requires significant labor costs when sites are remote or when images must be acquired frequently. Another calibration target is described in U.S. Pat. No. 6,191,851 (Kirkham et al.). Kirkham et al. discloses a calibration target that can be left in place adjacent to or in the field of interest to permit automatic calibration of the remote sensing system. However, the calibration target must still be deployed in or near the area to be imaged to provide the imagery characteristics of the target area in order to calibrate the data received by the remote sensor.
Accordingly, there is a need in the art of remote sensor technology to provide an inexpensive calibration method that can provide optical and thermal imagery characteristics without having to perform multiple calibration flights or travel during airborne or vehicular imaging applications. The aspects for cost reduction include equipment and material cost, mission execution and verification process, reduction of ground support tasks, efficiency and accessibility of deriving accurate position information from remotely sensed images.